The object of seismic prospecting operations is to record seismograms of the formation to be explored from elastic waves picked up by suitable receivers coupled with the formation (arranged at the surface or in wells), these waves being reflected by the subsoil discontinuities in response to waves emitted by an elastic wave source of any type, either an impulsive source: explosive charge in a hole, air guns towed by a ship at sea, etc., or vibrators emitting signals of variable length, generally variable-frequency signals. The frequency variation can be continuous within a certain frequency range (sweep) as described in patent U.S. Pat. No. 2,688,124 or discontinuous with binary coding as described in patent FR-2,589,587.
The vibrators can be, for example, of electromagnetic or electrohydraulic type, or piezoelectric. A piezoelectric type vibrator comprises for example a plate intended to provide coupling with the ground, a sufficiently heavy inertia mass coupled with the plate by means of one or more piezoelectric transducers. Each transducer comprises for example a pile of piezoelectric ceramic elements coupled in parallel, and it is connected to a vibrational signal generator. A piezoelectric vibrator is described for example in patent FR-2,791,780 filed by the applicant.
The seismic sources coupled with the ground surface are directional but the seismic energy they can emit depends very much on the coupling quality, which itself depends on the local climatic variations. This is a drawback notably when long-term monitoring operations are carried out in a reservoir under development so as to be able to compare, at intervals of several months for example, the seismograms obtained successively, and thus to detect variations in the state thereof. It is therefore preferable, when a certain emission reproducibility is desired, to couple the sources with the formation, below the weathered zone. A well of sufficient depth to reach the base of the weathered zone is therefore bored, the source is installed at the bottom thereof and connected to a suitable power generator.
However, the drawback of this coupling mode is that the source is no longer directional and emits upwards. The radiation that crosses the weathered zone disturbs the seismograms obtained.
A directional source is described, for an acoustic application, in patent U.S. Pat. No. 4,996,674. It is a high-frequency Janus type immersed source comprising a mass fastened between two piezoelectric transducers. A plate is fastened to the end of each transducer opposite the central mass. The mechanical impedance put up by the immersion medium is identical for each plate. The two transducers are fed independently of one another so that the motion of one or the other plate is inhibited. In the relatively high acoustic frequencies range, since the wavelengths of the waves emitted are short in view of the dimensions of the source, the waves are emitted only towards the outside of the mobile plate and practically not on the opposite side. In the very low frequencies range in which seismic sources operate, inhibition of the speed of one of the plates does not make the source directional because the mobile plate generates a back wave in phase opposition.